1. Field of the Invention
The present invention relates to a control circuit for a DC/DC converter, and more particularly to a control circuit for a DC/DC converter in which main switching means and sub switching means are alternately turned on and off so as to obtain a predetermined DC output voltage.
2. Description of the Related Art
DC/DC converters are widely used in applications in which output voltage of a DC power source is stepped down or up so as to obtain a predetermined DC output voltage; e.g., a power supply circuit for a cellular phone. In such a DC/DC converter, a switching element is turned on and off, and the on/off operation of the switching element is controlled in order to convert a DC input voltage to a predetermined DC output voltage. In general, a MOSFET is used as a switching element.
Among DC/DC converters of such a type, a step-down DC/DC converter has a reactor on the output side. Therefore, in periods during which the switching element is off, a closed circuit must be formed in order to discharge electrical energy accumulated in the reactor stemming from load current. Therefore, in the past, such a closed circuit has been realized by use of a recirculation diode.
However, when a recirculation diode is used, because of its relatively large forward voltage drop, the recirculation diode consumes a large amount of electrical power, thereby lowering the efficiency of the DC/DC converter. Such a large electrical power consumption becomes a serious problem when the DC/DC converter is realized in the form of an IC chip in order to reduce the size of the DC/DC converter.
In view of the above, there has been proposed a DC/DC converter in which a MOSFET, which serves as a switching element, is used in place of the above-described recirculation diode; a parasitic diode of the MOSFET is caused to function as a recirculation diode, and the switching function of the MOSFET is utilized so as to reduce the above-mentioned forward voltage drop. Notably, the on resistance of a MOSFET is much smaller than the forward-direction resistance of the recirculation diode, and the loss decreases accordingly.
Incidentally, in a DC/DC converter whose recirculation diode is replaced with a switching element, two switching elements generally formed of MOSFETs are mutually connected in series. Specifically, a first switching element (hereinafter referred to as a “main switching element”) for controlling the duty of on periods so as to regulate the output voltage to a desired value is connected in series with a second switching element (hereinafter referred to as a “sub switching element”) for discharging accumulated electrical energy from the above-mentioned reactor in periods during which the main switching element is off, and a DC output voltage is extracted from the connection point between the two switching elements via a reactor.
Therefore, in such a DC/DC converter, the main switching element and the sub switching element must be controlled in such a manner that these elements do not enter an on state simultaneously. In view of this, the on-off modes of the main switching element and the sub switching element are alternately switched, while a mode (blank period) in which the two switching elements are off is interposed between the on-off modes.
However, in such a DC/DC converter, when the sub switching element is turned off in a mode in which the main switching element is off and the sub switching element is on, there may occur a harmful reverse flow phenomenon in which current stemming from the electrical energy accumulated in the reactor flows to a DC power source via the parasitic diode of the main switching element. In order to avoid such a harmful reverse flow phenomenon, in a conventional DC/DC converter of the above-mentioned type, load current flowing through the sub switching element is monitored, and the sub switching element is turned off upon detection of a time point at which the load current becomes zero. This is because the reverse flow of load current occurs after the load current becomes zero.
Therefore, in a conventional DC/DC converter which includes a main switching element and a sub switching element that are alternately turned on and off so as to obtain a predetermined DC output voltage, a current detection circuit is provided in order to detect the above-mentioned load current (in particular, its flow direction). Such a current detection circuit includes a resistor which is connected in series to, for example, the sub switching element, and monitors the voltage produced across the resistor through comparison by use of a comparator. That is, the current detection circuit can detect, from the voltage produced across the resistor becoming zero, that the load current to be detected becomes zero.
Notably, Japanese Utility Model Application Laid-Open (kokai) No. 04-101286 discloses a DC/DC converter in which a main switching element and a sub switching element are alternately turned on and off so as to obtain a predetermined DC output voltage.
Incidentally, the above-described current detection circuit must detect, with high accuracy, a point in time at which the load current becomes zero. Therefore, in the case where the current detection circuit contains a comparator as described above, the comparator must be of high accuracy and is therefore expensive. Further, in the case where the current detection circuit is configured to provide high accuracy, current necessary to drive the current detection circuit increases, thereby hindering reduction in size of the DC/DC converter. Further, in the case where a resistor is connected in series to the switching element in order to form a current detection circuit, power loss is generated at the resistor, thereby hindering improvement in efficiency. These drawbacks become fatal in the case where such a DC/DC converter is fabricated in the form of an IC chip.